The calcium ion dependence of scallop myosin ATPase activity

Primary structure of myosin from the striated adductor muscle of the Atlantic scallop, Pecten maximus, and expression of the regulatory domain

We have determined the complete cDNA and deduced amino acid sequences of the heavy chain, regulatory light chain and essential light chain which constitute the molecular structure of myosin from the striated adductor muscle of the scallop, Pecten maximus. The deduced amino acid sequences of P. maximus regulatory light chain, essential light chain and heavy chain comprise 156, 156 and 1940 amino acids, respectively. These myosin peptide sequences, obtained from the most common of the eastern Atlantic scallops, are compared with those from three other molluscan myosins: the striated adductor muscles of Argopecten irradians and Placopecten magellanicus, and myosin from the siphon retractor muscle of the squid, Loligo pealei. The Pecten heavy chain sequence resembles those of the other two scallop sequences to a much greater extent as compared with the squid sequence, amino acid identities being 97.5% (A. irradians), 95.6% (P. magellanicus) and 73.6% (L. pealei), respectively. Myosin heavy chain residues that are known to be important for regulation are conserved in Pecten maximus. Using these Pecten sequences, we have overexpressed the regulatory light chain, and a combination of essential light chain and myosin heavy chain fragment, separately, in E. coli BL21 (DE3) prior to recombination, thereby producing Pecten regulatory domains without recourse to proteolytic digestion. The expressed regulatory domain was shown to undergo a calcium-dependent increase (approximately 7%) in intrinsic tryptophan fluorescence with a mid-point at a pCa of 6.6.

A folded (10 S) conformer of myosin from a striated muscle and its implications for regulation of ATPase activity

Myosin from the striated adductor muscle of the scallop Pecten maximus is shown to fold into a compact 10 S conformer under relaxing conditions, as has been characterized for smooth and non-muscle myosins. The folding transition is accompanied by the trapping of nucleotide at the active site to give a species with a half-life of about an hour at 20 degrees C. Ca2+ binding to the specific, regulatory sites on a myosin head promotes unfolding to the extended 6 S conformer and activates product release by 60-fold. The unfolding transition, however, remains much slower than the contraction-relaxation cycle of scallop striated muscle and could not play a role in the regulation of these events. The dissociation of products from myosin heads in native thick filaments is Ca2(+)-regulated, but under relaxing conditions the nucleotide is released at least an order of magnitude faster than from the 10 S monomeric myosin, at a rate similar to that observed with heavy meromyosin. Thus, there is no evidence for any intermolecular interaction between neighbouring molecules in the filament analogous to the head-neck intramolecular interaction in the 10 S conformer. It is possible that the 10 S myosin state represents an inert form involved in the control of filament assembly during muscle growth and development. Removal of regulatory light chains or labelling the reactive heavy chain thiol of myosin prevents, or at least disfavours, formation of the folded 10 S conformer and allows separation of the modified protein from the native molecules.